The present invention relates to a feedwater heater for use in heat power stations or nuclear power stations and, more particularly, to a feedwater heater of the type having a multiplicity of U-shaped heat transfer tubes and a vent tube for discharging the non-condensed gas in the heater.
Specifications of U.S. Pat. Nos. 4,254,825 and 4,219,077, assigned to the same assignee as this application, disclose a feedwater heater for the uses mentioned above. A brief description will be made as to the general construction of the feedwater heater common to these references. The feedwater heater of a horizontal type has a barrel or a shell, the inside of which is divided into sections by a tube plate which is normal to the axis of the shell. More specifically, at one end of the shell, a water chamber section is defined that consists of a water inlet chamber having a water inlet opening and a water outlet chamber having a water outlet opening. A plurality of heat transfer tubes are bent at their mid portions and are extended along the axis of the shell. These tubes are fixed to the tube plate such that one end of each tubes opens in the water inlet chamber, while the other end opens in the water outlet chamber. The heat transfer tubes are supported by a plurality of tube supporting plates, spaced at a suitable pitch in the longitudinal direction of the tubes. An inlet opening for steam and a drain are formed in the top of the shell. Also, a vent tube for discharging the non-condensed gas is disposed between the tube nest of the water inlet side, i.e. the lower tube nest, and the tube nest of the water outlet side, i.e. the uper tube nest, to permit the discharge of the non-condensed gas.
In operation, the feedwater coming into the feedwater heater from the water inlet chamber flows through the U-shaped heat transfer tubes and absorbs the heat from the heating steam coming into the feedwater heater from the steam inlet opening to condense the steam. The condensate is collected at the bottom of the shell and is discharged to the outside through a drain cooler which is located near the tube plate and surrounding the tube nest of the feedwater inlet side.
In the feedwater heater of this kind having more than one turn of the flow of feedwater, there is a large temperature difference of the feedwater between the tube nest of the feedwater inlet side and the tube nest of the feedwater outlet side. In consequence, the ratio of amount of heat exchanged in the inlet side tube nest to the amount of heat exchanged in the outlet side tube nest is as large as 20:1, so that a region of stagnation of steam is formed in the feedwater inlet side tube nest of the greater heat exchange. The non-condensed gas is inconveniently accumulated in this region. In consequence, the non-condensed gas is not concentrated to the vent tube and, hence, cannot be discharged smoothly, resulting in a corrosion of the heat transfer tubes in this region, as well as deteriorated transfer of the heat.